1. Field of the Invention
The present invention relates to a CR oscillation circuit which generates a clock having a frequency determined according to the resistance value and capacitance value.
2. Description of the Prior Art
FIG. 12 shows a structure of a conventional CR oscillation circuit. The CR oscillation circuit includes inverters 91, 92 and 93, a capacitive element 94, and a resistive element 95. This circuit outputs clock CLK which has a frequency determined according to a product of the capacitance value of the capacitive element 94 and the resistance value of the resistive element 95 (see, for example, Japanese Laid-Open Patent Publication No. 5-206731 and Japanese Laid-Open Patent Publication No. 7-131301).
Clock CLK generated by the CR oscillation circuit is used as, for example, a display control clock for driving display elements provided over a display panel, such as a liquid crystal panel, or the like. It has been generally known that, if the frequency of the display control clock is varied, the timing of driving the display elements is also varied, so that flickers are seen in a representation of the display panel.
However, to keep the frequency of clock CLK constant as much as possible, the capacitive element 94 is desirably formed using a material which exhibits only a small variation in capacitance value even in the case of a variation in the supply voltage (a material having a small voltage-dependence characteristic). For example, the interpolysilicon (polysilicon-to-polysilicon) capacitance or intermetal (metal-to-metal) capacitance is used as the capacitive element 94. The resistive element 95 is desirably formed using a material which exhibits only a small variation in resistance value even in the case of a variation in the supply voltage (a material having a small voltage-dependence characteristic) and which exhibits only a small variation in resistance value even in the case of a variation in temperature (a material having a small temperature-dependence characteristic). For example, a polysilicon resistance is used as the resistive element 95.
However, since the polysilicon resistance has a small temperature-dependence characteristic, the frequency of clock CLK varies according to a variation in the operating temperature or environmental temperature of a semiconductor integrated circuit device incorporating the CR oscillation circuit.
In the case of using the interpolysilicon capacitance or intermetal capacitance, an additional step for forming the capacitance in a semiconductor integrated circuit (an exclusive polysilicon formation step for forming a second layer capacitance or an exclusive metal formation step for forming an intermetal capacitance) is necessary. Thus, the cost of the semiconductor integrated circuit increases.
Transistors included in the inverters 91, 92 and 93 also have the voltage-dependence characteristic, and therefore, the frequency of clock CLK varies due to a variation in the supply voltage. As a result, flickers occur in a representation of the display.